Liquid crystal displays are used in the displays for everything from watches to computer screens and televisions. Most liquid crystal displays use twisted nematic liquid crystal materials. The twisted nematic liquid crystal displays include supertwist, double-supertwist, and monochrome supertwist which "twist" the light from 90 to 270 degrees. In these arrangements, the liquid crystal material is aligned by treating the surface of the substrate to which the material is applied. Usually, the substrate is glass or plastic which has an indium-tin-oxide (ITO) coating.
In twisted nematic liquid crystal displays, a white pixel is on and a black pixel is off. Ambient light passes through a layer of polarized glass (a polarizer) before the light passes into the aligned liquid crystal material. Uncharged nematic crystals, the most commonly used class of liquid crystal material, form a natural helix that twists at an angle which redirects the light so that it passes through a second polarizer. The light makes the pixel look white. When an electric charge is applied to the liquid crystal material, the liquid crystal molecules become aligned, so that the second polarizer no longer passes the light, making the pixel look black. In some liquid crystal displays the order is reversed, so that the pixel is on when the molecules are aligned.
Polymer-dispersed liquid crystal materials are just beginning to be used in display applications. Generally, the process of preparing these materials involves a phase separation in which a homogeneous solution of the polymer or prepolymer and the liquid crystal material is formed and the liquid crystal material later separates out of the solution as the final polymerization or solidification occurs.
The polymer-dispersed liquid crystal displays are advantageous in that the displays provide sufficient contrast to be used in ambient light conditions. In contrast, the twisted nematic liquid crystal displays are more difficult to read in ambient light. In addition to being capable of use in ambient light, the process of manufacturing polymer-dispersed liquid crystal displays is less expensive. The polymer-dispersed liquid crystal material can be simply applied to the substrate without any requirement for alignment as in the twisted nematic displays. In addition, no polarizers are required, further cutting the cost of the displays and increasing the amount of light passing through the material. The polarizers used in twisted nematic displays decrease the amount of light by at least forty percent.
Rather than using aligned liquid crystal materials as in the twisted nematic liquid crystal displays, the liquid crystal material is dispersed in a polymer matrix in which the liquid crystal material is randomly aligned. Therefore, light is scattered through the liquid crystal display when there is no power applied to the display, thereby minimizing the amount of power required to use the display and facilitating good contrast in ambient light conditions.
Although the power requirements are minimized in most liquid crystal displays, the polymer-dispersed liquid crystal displays generally require a driving voltage of sixty to eighty volts. Researchers have tried to lower the driving voltages by controlling the size of the droplets of liquid crystal material dispersed in the polymer matrix. In particular, the driving voltage requirements for some of the polymer-dispersed liquid crystal displays have been reportedly reduced to twenty to thirty volts by varying the droplet size.
Voltage drivers which supply the required driving voltage of sixty to eighty volts, or even twenty to thirty volts, are expensive in comparison to the voltage drivers required for nematic liquid crystal displays. Reducing the driving voltage to closer to the range of the twisted nematic liquid crystal displays (generally below ten volts) would allow wider use of the polymer-dispersed liquid crystal displays, facilitating their use in smaller, portable, battery-operated devices.